This invention relates to a regulator circuit for controlling the heat output of a heating element adapted to be switched on and off by a semiconductor switch energized by a zero crossing switch having regulator for control of periodical oscillations means, whereby a sensor signal derived from the sensed temperature is supplied to the zero crossing switch.
A circuit arrangement of this type takes advantage of the thermal inertia of heating elements, a property normally considered a liability. The heating up and cooling off of an electric heating element does not take place quickly, but requires a certain period of time, anywhere from seconds to minutes. If the electric power is supplied to the heating element in the form of pulse series from the main power supply, in which, for example, the number of switched on and switched off power supply fluctuations is varied, the medium value of the heat output can be fine tuned.
Since in this so-called regulator for control of periodical oscillations only semi-conductors are used for activating or deactivating the heating element, the life duration of the contact is immaterial. Moreover, with the selected control circuit, the switching time moves near the zero crossing of the power supply, so that at small voltages only small currents will pass. This is an additional safeguard to assure a long life of the switch.
The regulator for control of periodical oscillations is simply analogously realized in that a sensor signal is applied to an integrated circuit, known as a zero crossing switch, where it is superimposed upon a saw-tooth wave to control the ON time of the zero crossing switch independently of the sensor signal.
There are applications in which the heating element is installed in a hot plate, for example, to heat cookware, the temperature of the contents of which is being monitored. This type of heating system possesses a great deal of thermal inertia both in the heating phase and in the cooling phase. The result is that even if a circuit of the foregoing type is used, it is possible that the contents of the cooking vessel are overheated. This is simply due to the fact that after the heating element is turned off at the preset control temperature, the entire heating system continues to heat up because of its great inertia and may even reach a yet higher temperature. The danger of overheating also depends on the heating system, for example, the heat capacity of the heating element, the size of the cooking vessel and the volume of the contents of the vessel. This overheating at the end of the heating phase is undesirable also for reasons of energy inefficiency.